<script>
    var a = 0
    var a = 1
    console.log(a)
    let b = 2
    //let b = 3  // 重复定义
    console.log(b)

    {
        var c = 0
        let d = 1
    }

    console.log(c)
    // console.log(d)  // d is not defined
</script>
